Protocol to support adaptive station-dependent channel state information feedback rate in multi-user communication systems

ABSTRACT

Methods and apparatuses are proposed for supporting one or more user-dependent channel state information (CSI) feedback rates in a downlink spatial division multiple access (SDMA) system. For certain aspects, an access point (AP) may receive a channel evolution feedback from one or more stations and send a request for CSI to the stations whose CSI values need to be updated. For certain aspects, the AP may poll the stations for updated CSI values. For certain aspects, deterministic back-off timers may be assigned to the stations indicating when to send their CSI feedback. The proposed methods may improve system performance.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present application for patent claims benefit of U.S. ProvisionalPatent Application Ser. No. 61/243,891, entitled, “MAC Protocol toSupport Adaptive Station-Dependent Channel State Information Feedbackrate in Multi-User Communication Systems,” filed Sep. 18, 2009, and U.S.Provisional Patent Application Ser. No. 61/355,424, entitled, “MACProtocol to Support Adaptive Station-Dependent Channel State InformationFeedback rate in Multi-User Communication Systems,” filed Jun. 16, 2010,and U.S. Provisional Patent Application Ser. No. 61/358,234, entitled,“MAC Protocol to Support Adaptive Station-Dependent Channel StateInformation Feedback rate in Multi-User Communication Systems,” filedJun. 24, 2010, all assigned to the assignee hereof and hereby expresslyincorporated by reference herein.

FIELD

Certain aspects of the present disclosure generally relate to wirelesscommunications and, more particularly, to supporting an adaptivestation-dependent channel state information feedback rate in multi-usercommunication systems.

BACKGROUND

In order to address the issue of increasing bandwidth requirements thatare demanded for wireless communications systems, different schemes arebeing developed to allow multiple user terminals to communicate with asingle access point (AP) by sharing the channel resources whileachieving high data throughputs. Multiple Input or Multiple Output(MIMO) technology represents one such approach that has recently emergedas a popular technique for the next generation communication systems.MIMO technology has been adopted in several emerging wirelesscommunications standards such as the Institute of Electrical andElectronic Engineers (IEEE) 802.11 standard. IEEE 802.11 denotes a setof Wireless Local Area Network (WLAN) air interface standards developedby the IEEE 802.11 committee for short-range communications, forexample, tens of meters to a few hundred meters.

A MIMO wireless system employs a number (N_(T)) of transmit antennas anda number (N_(R)) of receive antennas for data transmission. A MIMOchannel formed by the N_(T) transmit and N_(R) receive antennas may bedecomposed into N_(S) spatial streams, where, for all practicalpurposes, N_(S)<=min {N_(T), N_(R)}. The N_(S) spatial streams may beused to transmit N_(S) independent data streams to achieve greateroverall throughput.

In wireless networks with a single access point and multiple stations,concurrent transmissions may occur on multiple channels toward differentstations, both in the uplink (UL) and downlink (DL) directions. Manychallenges are presented in such systems, such as the ability tocommunicate with legacy devices in addition to non-legacy devices,efficient use of resources, and interference.

SUMMARY

Certain aspects of the present disclosure provide a method for wirelesscommunications. The method generally includes transmitting a firstrequest message to request channel evolution feedback from a pluralityof apparatuses, receiving channel evolution feedback from theapparatuses, determining a subset of the apparatuses that shouldtransmit new channel state information (CSI) feedback based, at least inpart, on the channel evolution feedback, and transmitting a secondrequest message to the subset of apparatuses requesting new CSIfeedback.

Certain aspects of the present disclosure provide an apparatus forwireless communications. The apparatus generally includes a transmitterconfigured to transmit a first request message to request channelevolution feedback from a plurality of apparatuses, a receiverconfigured to receive channel evolution feedback from the apparatuses,circuit configured to determine a subset of the apparatuses that shouldtransmit new channel state information (CSI) feedback based, at least inpart, on the channel evolution feedback, and wherein the transmitter isfurther configured to transmit a second request message to the subset ofapparatuses requesting new CSI feedback.

Certain aspects of the present disclosure provide an apparatus forwireless communications. The apparatus generally includes means fortransmitting a first request message to request channel evolutionfeedback from a plurality of apparatuses, means for receiving channelevolution feedback from the apparatuses, means for determining a subsetof the apparatuses that should transmit new channel state information(CSI) feedback based, at least in part, on the channel evolutionfeedback, and wherein the means for transmitting further comprises meansfor transmitting a second request message to the subset of apparatusesrequesting new CSI feedback.

Certain aspects of the present disclosure provide a computer-programproduct for wireless communications, comprising a computer-readablemedium comprising instructions. The instructions executable fortransmitting a first request message to request channel evolutionfeedback from a plurality of apparatuses, receiving channel evolutionfeedback from the apparatuses, determining a subset of the apparatusesthat should transmit new channel state information (CSI) feedback based,at least in part, on the channel evolution feedback, and transmitting asecond request message to the subset of apparatuses requesting new CSIfeedback.

Certain aspects provide an access point for wireless communications. Theaccess point generally includes a plurality of antennas, a transmitterconfigured to transmit, via the plurality of antennas, a first requestmessage to request channel evolution feedback from a plurality ofapparatuses, a receiver configured to receive channel evolution feedbackfrom the apparatuses, circuit configured to determine a subset of theapparatuses that should transmit new channel state information (CSI)feedback based, at least in part, on the channel evolution feedback, andwherein the transmitter is further configured to transmit a secondrequest message to the subset of apparatuses requesting new CSIfeedback.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentdisclosure can be understood in detail, a more particular description,briefly summarized above, may be had by reference to aspects, some ofwhich are illustrated in the appended drawings. It is to be noted,however, that the appended drawings illustrate only certain typicalaspects of this disclosure and are therefore not to be consideredlimiting of its scope, for the description may admit to other equallyeffective aspects.

FIG. 1 illustrates a diagram of a wireless communications network inaccordance with certain aspects of the present disclosure.

FIG. 2 illustrates a block diagram of an example access point and userterminals in accordance with certain aspects of the present disclosure.

FIG. 3 illustrates a block diagram of an example wireless device inaccordance with certain aspects of the present disclosure.

FIG. 4 illustrates a two-step medium access control (MAC) protocol forheterogeneous channel state information (CSI) feedback, in accordancewith certain aspects of the present disclosure.

FIG. 5 illustrates example operations that may be performed by an accesspoint for a two-step MAC protocol with heterogeneous CSI feedback, inaccordance with certain aspects of the present disclosure.

FIG. 5A illustrates example components capable of performing theoperations shown in FIG. 5.

FIG. 6 illustrates example operations that may be performed by a stationfor a two-step MAC protocol with heterogeneous CSI feedback, inaccordance with certain aspects of the present disclosure.

FIG. 6A illustrates example components capable of performing theoperations shown in FIG. 6.

FIG. 7 illustrates a MAC protocol with heterogeneous CSI feedback basedon deterministic back-off timers, in accordance with certain aspects ofthe present disclosure.

FIG. 8 illustrates example operations that may be performed by an accesspoint for a MAC protocol with heterogeneous CSI feedback based ondeterministic back-off timers, in accordance with certain aspects of thepresent disclosure.

FIG. 8A illustrates example components capable of performing theoperations shown in FIG. 8.

FIG. 9 illustrates example operations that may be performed by a stationfor a MAC protocol with heterogeneous CSI feedback based ondeterministic back-off timers, in accordance with certain aspects of thepresent disclosure.

FIG. 9A illustrates example components capable of performing theoperations shown in FIG. 9.

FIG. 10 illustrates a MAC protocol with heterogeneous CSI feedback basedon polling of stations, in accordance with certain aspects of thepresent disclosure.

FIG. 11 illustrates example operations that may be performed by anaccess point for a MAC protocol with heterogeneous CSI feedback based onpolling of stations, in accordance with certain aspects of the presentdisclosure.

FIG. 11A illustrates example components capable of performing theoperations shown in FIG. 11.

FIG. 12 illustrates example operations that may be performed by astation for a MAC protocol with heterogeneous CSI feedback based onpolling of stations, in accordance with certain aspects of the presentdisclosure.

FIG. 12A illustrates example components capable of performing theoperations shown in FIG. 12.

FIG. 13 illustrates example operations that may be performed by anaccess point for a MAC protocol with heterogeneous CSI feedback, inaccordance with certain aspects of the present disclosure.

FIG. 13A illustrates example components capable of performing theoperations shown in FIG. 13.

FIG. 14 illustrates example operations that may be performed by astation for a MAC protocol with heterogeneous CSI feedback, inaccordance with certain aspects of the present disclosure.

FIG. 14A illustrates example components capable of performing theoperations shown in FIG. 14.

DETAILED DESCRIPTION

Various aspects of certain aspects of the present disclosure aredescribed below. It should be apparent that the teachings herein may beembodied in a wide variety of forms and that any specific structure,function, or both being disclosed herein is merely representative. Basedon the teachings herein one skilled in the art should appreciate that anaspect disclosed herein may be implemented independently of any otheraspects and that two or more of these aspects may be combined in variousways. For example, an apparatus may be implemented or a method may bepracticed using any number of the aspects set forth herein. In addition,such an apparatus may be implemented or such a method may be practicedusing other structure, functionality, or structure and functionality inaddition to or other than one or more of the aspects set forth herein.Furthermore, an aspect may comprise at least one element of a claim.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any aspect described herein as “exemplary”is not necessarily to be construed as preferred or advantageous overother aspects. Also as used herein, the term “legacy stations” generallyrefers to wireless network nodes that support 802.11n or earlierversions of the Institute of Electrical and Electronics Engineers (IEEE)802.11 standard.

The multi-antenna transmission techniques described herein may be usedin combination with various wireless technologies such as Code DivisionMultiple Access (CDMA), Orthogonal Frequency Division Multiplexing(OFDM), Time Division Multiple Access (TDMA), Spatial Division MultipleAccess (SDMA), and so on. Multiple user terminals can concurrentlytransmit/receive data via different (1) orthogonal code channels forCDMA, (2) time slots for TDMA, or (3) sub-bands for OFDM. A CDMA systemmay implement IS-2000, IS-95, IS-856, Wideband-CDMA (W-CDMA) or someother standards. An OFDM system may implement IEEE 802.11 or some otherstandards. A TDMA system may implement GSM or some other standards.These various standards are known in the art.

An Example MIMO System

FIG. 1 illustrates a multiple-access MIMO system 100 with access pointsand user terminals. For simplicity, only one access point 110 is shownin FIG. 1. An access point (AP) is generally a fixed station thatcommunicates with the user terminals and may also be referred to as abase station or some other terminology. A user terminal may be fixed ormobile and may also be referred to as a mobile station, a station (STA),a client, a wireless device or some other terminology. A user terminalmay be a wireless device, such as a cellular phone, a personal digitalassistant (PDA), a handheld device, a wireless modem, a laptop computer,a personal computer, etc.

Access point 110 may communicate with one or more user terminals 120 atany given moment on the downlink and uplink. The downlink (i.e., forwardlink) is the communication link from the access point to the userterminals, and the uplink (i.e., reverse link) is the communication linkfrom the user terminals to the access point. A user terminal may alsocommunicate peer-to-peer with another user terminal. A system controller130 couples to and provides coordination and control for the accesspoints.

System 100 employs multiple transmit and multiple receive antennas fordata transmission on the downlink and uplink. Access point 110 isequipped with a number N_(ap) of antennas and represents themultiple-input (MI) for downlink transmissions and the multiple-output(MO) for uplink transmissions. A set N_(u) of selected user terminals120 collectively represents the multiple-output for downlinktransmissions and the multiple-input for uplink (UL) transmissions. Incertain cases, it may be desirable to have N_(ap)≧N_(u)≧1 if the datasymbol streams for the N_(u) user terminals are not multiplexed in code,frequency or time by some means. N_(u) may be greater than N_(ap) if thedata symbol streams can be multiplexed using different code channelswith CDMA, disjoint sets of sub-bands with OFDM, and so on. Eachselected user terminal transmits user-specific data to and/or receivesuser-specific data from the access point. In general, each selected userterminal may be equipped with one or multiple antennas (i.e., N_(ut)≧1).The N_(u) selected user terminals can have the same or different numberof antennas.

MIMO system 100 may be a time division duplex (TDD) system or afrequency division duplex (FDD) system. For a TDD system, the downlinkand uplink share the same frequency band. For an FDD system, thedownlink and uplink use different frequency bands. MIMO system 100 mayalso utilize a single carrier or multiple carriers for transmission.Each user terminal may be equipped with a single antenna (e.g., in orderto keep costs down) or multiple antennas (e.g., where the additionalcost can be supported).

FIG. 2 shows a block diagram of access point 110 and two user terminals120 m and 120 x in MIMO system 100. Access point 110 is equipped withN_(ap) antennas 224 a through 224 ap. User terminal 120 m is equippedwith N_(ut,m) antennas 252 ma through 252 mu, and user terminal 120 x isequipped with N_(ut,x) antennas 252 xa through 252 xu. Access point 110is a transmitting entity for the downlink and a receiving entity for theuplink. Each user terminal 120 is a transmitting entity for the uplinkand a receiving entity for the downlink. As used herein, a “transmittingentity” is an independently operated apparatus or device capable oftransmitting data via a frequency channel, and a “receiving entity” isan independently operated apparatus or device capable of receiving datavia a frequency channel. In the following description, the subscript“dn” denotes the downlink, the subscript “up” denotes the uplink, N_(up)user terminals are selected for simultaneous transmission on the uplink,N_(dn) user terminals are selected for simultaneous transmission on thedownlink, N_(up) may or may not be equal to N_(dn), and N_(up) andN_(dn) may be static values or can change for each scheduling interval.The beam-steering or some other spatial processing technique may be usedat the access point and user terminal.

On the uplink, at each user terminal 120 selected for uplinktransmission, a TX data processor 288 receives traffic data from a datasource 286 and control data from a controller 280. TX data processor 288processes (e.g., encodes, interleaves, and modulates) the traffic data{d_(up,m)} for the user terminal based on the coding and modulationschemes associated with the rate selected for the user terminal andprovides a data symbol stream {s_(up,m)}. A TX spatial processor 290performs spatial processing on the data symbol stream {s_(up,m)} andprovides N_(ut,m) transmit symbol streams for the N_(ut,m) antennas.Each transmitter unit (TMTR) 254 receives and processes (e.g., convertsto analog, amplifies, filters, and frequency upconverts) a respectivetransmit symbol stream to generate an uplink signal. N_(ut,m)transmitter units 254 provide N_(ut,m) uplink signals for transmissionfrom N_(ut,m) antennas 252 to the access point 110.

A number N_(up) of user terminals may be scheduled for simultaneoustransmission on the uplink. Each of these user terminals performsspatial processing on its data symbol stream and transmits its set oftransmit symbol streams on the uplink to the access point.

At access point 110, N_(ap) antennas 224 a through 224 ap receive theuplink signals from all N_(up) user terminals transmitting on theuplink. Each antenna 224 provides a received signal to a respectivereceiver unit (RCVR) 222. Each receiver unit 222 performs processingcomplementary to that performed by transmitter unit 254 and provides areceived symbol stream. An RX spatial processor 240 performs receiverspatial processing on the N_(ap) received symbol streams from N_(ap)receiver units 222 and provides N_(up) recovered uplink data symbolstreams. The receiver spatial processing is performed in accordance withthe channel correlation matrix inversion (CCMI), minimum mean squareerror (MMSE), successive interference cancellation (SIC), or some othertechnique. Each recovered uplink data symbol stream {s_(up,m)} is anestimate of a data symbol stream {s_(up,m)} transmitted by a respectiveuser terminal. An RX data processor 242 processes (e.g., demodulates,deinterleaves, and decodes) each recovered uplink data symbol stream{s_(up,m)} in accordance with the rate used for that stream to obtaindecoded data. The decoded data for each user terminal may be provided toa data sink 244 for storage and/or a controller 230 for furtherprocessing.

On the downlink, at access point 110, a TX data processor 210 receivestraffic data from a data source 208 for N_(dn) user terminals scheduledfor downlink transmission, control data from a controller 230 andpossibly other data from a scheduler 234. The various types of data maybe sent on different transport channels. TX data processor 210 processes(e.g., encodes, interleaves, and modulates) the traffic data for eachuser terminal based on the rate selected for that user terminal. TX dataprocessor 210 provides N_(dn) downlink data symbol streams for theN_(dn) user terminals. A TX spatial processor 220 performs spatialprocessing on the N_(dn) downlink data symbol streams, and providesN_(ap) transmit symbol streams for the N_(ap) antennas. Each transmitterunit (TMTR) 222 receives and processes a respective transmit symbolstream to generate a downlink signal. N_(ap) transmitter units 222provide N_(ap) downlink signals for transmission from N_(ap) antennas224 to the user terminals.

At each user terminal 120, N_(ut,m) antennas 252 receive the N_(ap)downlink signals from access point 110. Each receiver unit (RCVR) 254processes a received signal from an associated antenna 252 and providesa received symbol stream. An RX spatial processor 260 performs receiverspatial processing on N_(ut,m) received symbol streams from N_(ut,m)receiver units 254 and provides a recovered downlink data symbol stream{s_(dn,m)} for the user terminal. The receiver spatial processing isperformed in accordance with the CCMI, MMSE or some other technique. AnRX data processor 270 processes (e.g., demodulates, deinterleaves, anddecodes) the recovered downlink data symbol stream to obtain decodeddata for the user terminal.

FIG. 3 illustrates various components that may be utilized in a wirelessdevice 302 that may be employed within the system 100. The wirelessdevice 302 is an example of a device that may be configured to implementthe various methods described herein. The wireless device 302 may be anaccess point 110 or a user terminal 120.

The wireless device 302 may include a processor 304 which controlsoperation of the wireless device 302. The processor 304 may also bereferred to as a central processing unit (CPU). Memory 306, which mayinclude both read-only memory (ROM) and random access memory (RAM),provides instructions and data to the processor 304. A portion of thememory 306 may also include non-volatile random access memory (NVRAM).The processor 304 typically performs logical and arithmetic operationsbased on program instructions stored within the memory 306. Theinstructions in the memory 306 may be executable to implement themethods described herein.

The wireless device 302 may also include a housing 308 that may includea transmitter 310 and a receiver 312 to allow transmission and receptionof data between the wireless device 302 and a remote location. Thetransmitter 310 and receiver 312 may be combined into a transceiver 314.A plurality of transmit antennas 316 may be attached to the housing 308and electrically coupled to the transceiver 314. The wireless device 302may also include (not shown) multiple transmitters, multiple receivers,and multiple transceivers.

The wireless device 302 may also include a signal detector 318 that maybe used in an effort to detect and quantify the level of signalsreceived by the transceiver 314. The signal detector 318 may detect suchsignals as total energy, energy per subcarrier per symbol, powerspectral density and other signals. The wireless device 302 may alsoinclude a digital signal processor (DSP) 320 for use in processingsignals.

The various components of the wireless device 302 may be coupledtogether by a bus system 322, which may include a power bus, a controlsignal bus, and a status signal bus in addition to a data bus.

Those skilled in the art will recognize the techniques described hereinmay be generally applied in systems utilizing any type of multipleaccess schemes, such as SDMA, OFDMA, CDMA, SDMA and combinationsthereof.

Protocol to Support Adaptive Station-Dependent Channel State InformationFeedback Rate in Multi-User Communication Systems

Certain aspects of the present disclosure provide methods for supportingone or more user-dependent channel state information (CSI) feedbackrates in a downlink SDMA system. For certain aspects, the access point(AP) may receive channel evolution feedbacks from one or more stationsand send a request for CSI to the stations whose CSI values need to beupdated. For certain aspects, the AP may poll the stations for updatedCSI values. For certain aspects, deterministic back-off timers may beassigned to the stations indicating when to send their CSI feedback. Theproposed methods may improve system performance.

A system utilizing downlink multi-user MIMO (MU-MIMO) or SDMA maysimultaneously serve multiple spatially separated stations bytransmit-beamforming from an antenna array at the AP. The AP maycalculate complex transmit pre-coding weights based on channel stateinformation received from each of the stations.

Since the channel varies with time due to station mobility or modestirring by objects moving in the environment, the CSI should be updatedperiodically in order for the AP to beamform accurately to each station.The required rate of CSI feedback for each station may depend on thecoherence time of the channel between the AP and that station. Aninsufficient feedback rate may adversely impact performance due toinaccurate beamforming. On the other hand, an excessive feedback ratemay produce minimal additional benefit, while wasting valuable mediumtime.

In a scenario consisting of multiple spatially separated stations, itmay be expected that the channel coherence time, and therefore theappropriate CSI feedback rate, varies spatially across stations. Inaddition, due to various factors such as changing channel conditions andmobility of the stations, the appropriate CSI feedback rate may alsovary temporally for each of the stations.

For example, some stations such as a high definition television (HDTV)or a set-top box are stationary, whereas other stations such as handhelddevices are subject to motion. Furthermore, a subset of stations may besubject to high Doppler from fluorescent light effects. Finally,multi-paths to some stations may have more Doppler than others sincedifferent scatterers may move at different velocities and affectdifferent subsets of stations.

Therefore, if a single rate of CSI feedback is used for all thestations, system performance may suffer due to inaccurate beamformingfor stations with insufficient feedback rates and/or excessive feedbackoverhead for stations with unnecessarily high feedback rates.

In conventional schemes, the CSI feedback may occur at a rate consistentwith the worst-case station in terms of mobility or temporal channelvariation. However, for an SDMA system consisting of stations thatexperience a range of channel conditions, a single CSI feedback rate maynot be appropriate for all the stations. Catering to the worst-casestation may result in an unnecessary waste of channel resources byforcing the stations that experience relatively static channelconditions to feedback CSI values with the same rate as those in highlydynamic channels.

For example, in the CDMA2000 standard, in Evolution-Data Optimized(EV-DO) data rate control channel (DRC), the channel state informationreflects the signal to interference plus noise ratio (SINR) of thereceived pilot. In addition, the channel state information is sent bythe station to facilitate rate selection for the next transmission. Thisinformation is updated at a fixed rate for all of the stations,presumably at a rate sufficient to track channel variations associatedwith the worst-case expected mobility situations. It is likely that thisrate of channel state feedback is unnecessarily high for staticstations. It should be noted that the DRC was designed to provideminimal overhead. Because CSI in an SDMA system is used to supportcomplex beamforming at the AP, it may not be feasible to compress orstreamline the CSI feedback to the degree accomplished in the EV-DOdesign.

As a second example, the IEEE 802.11n standard, which supports transmitbeamforming, does not specify a rate at which CSI feedback should besent. Therefore, the CSI feedback rate may be considered animplementation factor. In contrast, due to the potentially high overheadof CSI feedback for multiple SDMA stations in the IEEE 802.11acstandard, and the potential for abuse of such CSI feedback mechanism byrogue stations, it may be necessary to specify these protocols in thestandard specification.

As described above, an appropriate rate of CSI feedback for a particularstation may depend on signal to noise ratio (SNR) conditions of thestation. For certain aspects, stations with lower SNR values, and hencelower downlink modulation and coding scheme (MCS) levels, may be biasedtoward a lower CSI feedback rate. Throughput penalty due to precodingbased on staled CSI may be smaller for low MCS/SNR stations than thepenalty for high MCS/SNR stations. In addition, uplink resourcesrequired to communicate CSI by the stations with low MCS (e.g., with lowdata rate) may be larger than the resources required by the stationsthat experience high SNR conditions. Therefore, for certain aspects,low-SNR stations may completely be excluded from downlink multi-user(MU)-MIMO.

Certain aspects of the present disclosure propose a medium accesscontrol (MAC) layer protocol that allows user-dependent andtime-dependent CSI feedback transmissions. In the proposed MAC layerprotocol, each station in a multi-user MIMO system sends CSI at a rateappropriate to its channel conditions. The proposed protocol may lead tosubstantial improvements in network throughput and channel efficiency.

FIG. 4 illustrates a two-step MAC protocol with heterogeneous CSIfeedback, in accordance with certain aspects of the present disclosure.In the first step, the access point may request channel evolutionfeedback (CEFB, 406) from one or more stations. In the second step, theAP may request CSI feedback 410 from a subset of stations. The AP maydecide to request feedback from a subset of stations based on the degreeof channel evolution of each station, SNR or MCS values of each station,and the overall expected interference level in the next SDMAtransmission. The proposed method may allow significant reduction offeedback overhead by exploiting uplink SDMA.

For certain aspects, the transaction illustrated in FIG. 4 may beinitiated by the AP using a training request message (TRM) 402. The TRMmessage may be transmitted using the lowest supported rate with a formatdecodable by legacy IEEE 802.11a/g stations. The TRM message may servetwo purposes. First, the TRM may be utilized for requesting channelevolution data from all the stations or a subset of stations. Forexample, the subset of stations may be candidates for an impendingdownlink SDMA transmission. Second, the TRM message may be used forprotecting the channel evolution feedback transmission. For example, theinformation in the duration field of the TRM message may be used by allthe non-participating stations to set their network allocation vector(NAV) appropriately to minimize interference.

The payload of the TRM message may contain bits to indicate a requestfor channel evolution (i.e., channel state information request).Following a short inter-frame space (SIFS) interval, the AP may transmita Null Data Packet (NDP) 404 containing a very high throughput (VHT)preamble to the stations. The NDP message may be used for downlinkchannel sounding. Unlike the TRM, the NDP message may not be decodableby legacy stations. Each station may respond to the combination of theTRM and NDP messages with a CEFB message 406, which may contain a metricor metrics indicating degree of channel aging since the most recent CSIwas sent.

The AP may use the metrics received from each station, as well as othernetwork status parameters such as the total number of SDMA stations,their MCS and transmit power to send a second TRM message 408. Thesecond TRM message 408 may be used to request channel feedback from asubset of stations whose CSI needs to be updated. This TRM message mayalso specify the MCS at which each station shall send its CSI feedbackmessage. After receiving the second TRM message, the stations mayrespond with their CSI feedback messages. The duration field of thesecond TRM message 408 may be set to protect the entire duration of CSIfeedback transmission from interference caused by non-participatingstations, including legacy stations.

The AP may update its precoding weights based on the received CSIfeedback and transmit downlink SDMA data 412 to the stations. Forcertain aspects, the downlink SDMA data transmission may be protected bya clear to send (CTS)-to-Self message. The CTS-to self message may betransmitted before the SDMA data transmission to reserve the medium forthe data transmission. The CTS message may also be protected by theduration field in the second TRM message 408.

If a system supports uplink SDMA (UL-SDMA), simultaneous transmission ofCEFB or CSI messages utilizing UL-SDMA from all the stations may be themost efficient implementation of the proposed protocol illustrated inFIG. 4. However, in the absence of UL-SDMA, CEFB and CSI messages may betransmitted serially by time division multiple access (TDMA) ororthogonal frequency division multiple access (OFDMA).

FIG. 5 illustrates example operations 500 that may be performed by anaccess point for a two-step MAC protocol with heterogeneous CSIfeedback, in accordance with certain aspects of the present disclosure.At 502, the access point may transmit a first request message to requestchannel evolution feedback from a plurality of apparatuses (e.g.,stations), wherein the channel evolution feedback indicates a degree ofchannel aging since a most recent CSI update. At 504, the access pointmay receive channel evolution feedback from the apparatuses.

At 506, the access point may determine a subset of the apparatuses thatshould update their CSI feedback based, at least in part, on the channelevolution feedback. At 508, the access point may transmit a secondrequest message to the subset of apparatuses requesting CSI feedback.For certain aspects, the first or second request messages may be nulldata packet (NDP) announcement frames.

For certain aspects, the CSI feedback 410 may comprise a representationof a current estimated channel, or a relative change in an estimatedchannel since previously received CSI feedback was transmitted. Forcertain aspects, the first request message 402 may indicate a type ofCSI feedback being supported by the access point. For example, the firstrequest message may indicate whether differential updates to CSIfeedback are supported by the access point. For certain aspects, type ofCSI feedback may be multi-user (MU) or single user (SU).

For certain aspects, the AP may determine rate of channel evolution orDoppler at a subset of stations in order to determine the subset ofstations that should transmit new CSI feedback. For certain aspects, theAP may compare the channel evolution feedback with a previously obtainedchannel evolution feedback to determine the subset of stations that needto send a new CSI feedback.

FIG. 6 illustrates example operations 600 that may be performed by astation for a two-step MAC protocol with heterogeneous CSI feedback, inaccordance with certain aspects of the present disclosure. At 602, thestation may receive a first request message from an apparatus (e.g., anaccess point), the request message requesting channel evolutionfeedback. At 604, the station may generate channel evolution feedbackbased on one or more signals received from the apparatus, wherein thechannel evolution feedback indicates a degree of channel aging since amost recent channel state information (CSI). At 606, the station maytransmit the channel evolution feedback to the apparatus in response tothe first request message.

For certain aspects, if CSI feedback is not accomplished by UL-SDMA, theduration field contained in the second TRM message may be calculated bythe AP with the assumption that all the stations will send CSI feedback.This mechanism may protect the CEFB and CSI messages from collisionsoccurring due to transmissions from the stations that are notparticipating in feedback transmissions.

For certain aspects, a ‘soft’ channel evolution metric may be used thatcentralizes the decision to request CSI at the AP. The AP may alsoconsider other factors such as the multi-user interference level and MCSof each station in the decision.

FIG. 7 illustrates an alternative MAC protocol with heterogeneous CSIfeedback based on deterministic back-off timers, in accordance withcertain aspects of the present disclosure. As illustrated, the decisionto transmit a CSI feedback message may be performed in a single step. Inaddition, each of the stations may decide whether or not to transmit aCSI feedback to the AP. The decision may be based, at least in part, ondefined metric and predetermined criteria. Only the stations whichdetermine that the channel has changed since the last time a CSIfeedback message was sent may transmit CSI feedback. As a result, theCSI feedback overhead may be reduced.

The protocol illustrated in FIG. 7 may be more appropriate for airinterfaces in which UL-SDMA is not available. In the proposed protocol,each SDMA station may decide weather or not to transmit CSI feedbackbased on an internal calculation akin to a hard metric. Timing of theserial CSI transmissions may be accomplished by exploiting adeterministic back-off timer.

The AP may initiate the transactions in FIG. 7 by transmitting a TRMmessage addressed to those stations intended for a pending DL-SDMAtransmission. The TRM message may contain a deterministic back-offassignment for each station. Similar to FIG. 4, the TRM message may befollowed by an NDP message providing a sounding preamble. Each stationmay respond in turn with CSI feedback if the station decides a CSIupdate is needed at the AP. If a station decides CSI update is notrequired, the station may not transmit anything.

In order to minimize collisions in the CSI feedback messages transmittedby different station, each station may utilize a deterministic back-offtimer. Each station may only transmit when its back-off timer expires.Each station may also pause its timer if the station detectstransmission by another station. Timers may resume counting down afterthe other station completes its transmission and vacates the medium. Theback-off values may be selected to minimize the amount of time that maybe lost due to non-responding stations. Reducing the lost time may helpreduce the total time required to receive all the CSI feedback messages.

Following the reception of a CSI message from the last station, or theexpiration of the longest back-off timer, the AP may recalculateprecoding weights and start DL-SDMA transmission 412. In the exampleillustrated in FIG. 7, STA3 does not transmit a CSI feedback message,and STA4 begins transmitting a CSI feedback message after a minimaldelay.

For certain aspects of the present disclosure, the request message mayprovide an indication that the CSI needs to be sent using a soundingframe or a data frame.

FIG. 8 illustrates example operations 800 that may be performed by anaccess point for a MAC protocol with heterogeneous CSI feedback based ondeterministic back-off timers, in accordance with certain aspects of thepresent disclosure. At 802, the access point transmits a request messageto a plurality of apparatuses to request channel state information (CSI)feedback, wherein the request message provides a deterministic back-offtimer assignment for each of the apparatuses indicating when to sendtheir CSI feedback. At 804, the access point receives, from one or moreof the apparatuses, CSI feedback transmitted in accordance with theback-off timer assignments.

FIG. 9 illustrates example operations 900 that may be performed by astation for a MAC protocol with heterogeneous CSI feedback based ondeterministic back-off timers, in accordance with certain aspects of thepresent disclosure. At 902, the station receives a request message froman apparatus requesting channel state information (CSI) feedback,wherein the request comprises a deterministic back-off timer indicatingwhen to transmit the CSI feedback. At 904, the station transmits the CSIfeedback in accordance with the back-off timer.

One disadvantage of this protocol is that the deterministic back-offconcept assumes all the stations can detect the transmissions of theother stations by sensing the medium. However, in the presence of hiddennodes, back-off timers may not pause as expected, potentially leading tocollisions of CSI feedback data.

FIG. 10 illustrates a MAC protocol with heterogeneous CSI feedback basedon polling of stations, in accordance with certain aspects of thepresent disclosure. This protocol avoids the hidden node problem andhence avoids collision of the transmissions from different stations byutilizing a polling protocol.

As illustrated in FIG. 10, following transmission of the TRM andsounding NDP messages, each station may be polled sequentially for CSIfeedback. A station may respond to polling 1002 by transmitting CSIfeedback if the station determines that a CSI update is required.Otherwise, the station may not transmit anything. If the AP does notdetect a response to a poll after one timeslot, the AP polls the nextstation. Following the reception of CSI from the last station, or noresponse from the final polled station, the AP may recalculate theprecoding weights and may begin DL-SDMA data transmission. In theexample illustrated in FIG. 10, STA3 does not transmit a CSI feedbackmessage. When the AP does not detect a response from STA3 in a certaintime, the AP may poll STA4 for CSI feedback.

FIG. 11 illustrates example operations 1100 that may be performed by anaccess point for a MAC protocol with heterogeneous CSI feedback based onpolling, in accordance with certain aspects of the present disclosure.At 1102, the access point transmits separate polling messages to polleach of a plurality of apparatuses (e.g., stations) for channel stateinformation (CSI) feedback. At 1104, in response to the pollingmessages, the access point receives CSI feedback from one or more of thepolled apparatuses.

For certain aspects, the polling messages may be preceded by an NDPannouncement frame followed by an NDP frame. For certain aspects,polling messages transmitted to a station with slowly evolving channelmay be less frequent, compared to polling messages transmitted to astation with faster evolving channel.

FIG. 12 illustrates example operations 1200 that may be performed by astation for a MAC protocol with heterogeneous CSI feedback based onpolling, in accordance with certain aspects of the present disclosure.At 1202, the station determines if channel state information (CSI) needsto be updated. At 1204, the station receives a polling message from anapparatus (e.g., an access point). At 1206, the station transmits CSIfeedback to the apparatus in response to the polling message, if it isdetermined that the CSI needs to be updated.

For certain aspects, the station may calculate a CSI value based on thesignals received from an access point. The station may compare the CSIvalue with a most recent CSI update that was transmitted to the AP. Thestation may decide to update CSI if a difference between the CSI valueand the most recent CSI value is equal to or larger than a thresholdvalue.

For certain aspects of the present disclosure, the TRM message may havea format that is decodable by legacy devices (i.e., the stations that donot support DL-SDMA). Therefore, the TRM message may be decoded by allthe stations, even the legacy stations. The TRM message may carry aduration field so that some of the stations defer their transmissions bysetting their NAV appropriately. The stations who defer theirtransmissions may be the stations that are not taking part in theupcoming DL-SDMA transmission or stations that are not capable of SDMA.

For certain aspects, the duration field contained in the TRM message maybe calculated by the AP assuming that all of the stations may feedbackCSI messages. The duration field of the TRM message may be used toprotect the sounding NDP and CSI messages from collisions occurring dueto transmissions of stations not participating in feedbacktransmissions.

The present disclosure proposed protocols to reduce the CSI feedbackoverhead when uplink SDMA is supported. Certain aspects may also reducefeedback overhead when UL-SDMA is not supported. As described in thedocument, the channel evolution and CSI feedback may be protected fromdata collisions by informing the legacy stations, or other stations thatare not participating in any specific SDMA transmission, about theupcoming feedback transmissions.

CSI Reporting Options

As described above, certain aspects of the present disclosure allow anAP to receive CSI from multiple stations. The CSI information may besent based on a degree of channel evolution.

According to certain aspects, an AP may transmit a request message, suchas a sounding message to a set of stations, allowing them to estimatethe channel. According to certain aspects, the request message mayinclude an indication of a kind of CSI report the AP may be able toaccept. As an example, to support differential CSI updates, an AP may berequired to store a previous CSI report, which some APs may not be ableto do.

In any case, each station may estimate the channel based on the message.Each station may calculate a difference between the estimated channeland a previously estimated channel that may be stored in the memory.Each station may also calculate a metric base on the difference. Eachstation may reply with a message based on the calculated metric. Forexample, the message may have one of the following types: a full CSIreport, a Null or Acknowledgement (ACK) frame, or a differential CSIreport. The full CSI report may be a packet with complete CSI, quantizedwith full resolution. The Null or ACK frame may be a packet containingno CSI, indicating the channel has not changed significantly since aprevious CSI transmission. The differential CSI report may be aquantized difference of CSI with respect to the previous CSI report,quantized with a number of bits smaller than the number of bits used forfull CSI report.

According to certain aspects, a CSI reply message may also indicate thetype of CSI message (e.g., full CSI report or differential CSI report)and the quantization parameters. According to certain aspects, thequantization parameters may be defined a priori, via an alternativemessaging scheme. Replies from the stations may follow any of theschemes described above such as sequential, using back-off timer orpolled.

FIG. 13 illustrates example operations that may be performed by anaccess point for a MAC protocol with heterogeneous CSI feedback, inaccordance with certain aspects of the present disclosure. Asillustrated, at 1302, an access point may transmit a first requestmessage to one or more apparatuses (e.g., stations) requesting CSIfeedback. At 1304, the access point may receive first CSI feedback fromat least one of the apparatuses, wherein the first CSI feedbackcomprises at least one of a Null data frame or an acknowledgement frameto indicate an estimated channel has not significantly changed since CSIfeedback was last transmitted.

For certain aspects the first request message may be an NDP announcementframe. The first request message may also be followed by an NDP frameand a poll frame. The stations may use the NDP frame to estimate thechannel. As described earlier, the polling message may notify thestations to send a CSI update to the AP at a certain time. The firstrequest message may also indicate whether the AP supports differentialupdates to CSI.

For certain aspects, the AP may transmit a second request message to thestations requesting CSI feedback. The AP may then receive a second CSIfeedback from at least one of the stations. The second CSI feedback mayinclude a representation of a relative change in an estimated channelsince previously received CSI feedback was transmitted.

For certain aspects, the AP may update precoding weights used fortransmissions to a subset of stations, based on the received CSIfeedback. For example, the AP may update the precoding weights for thestations from which it has received an updated CSI value.

FIG. 14 illustrates example operations that may be performed by astation for a MAC protocol with heterogeneous CSI feedback, inaccordance with certain aspects of the present disclosure. Asillustrated, at 1402, a station may receive a first request message froman apparatus (e.g., an access point), the first request messagerequesting channel state information (CSI) feedback. At 1404, thestation may transmit a first CSI feedback message to the apparatus inresponse to the first request message wherein the first CSI feedbackmessage comprises at least one of a Null data frame or anacknowledgement frame to indicate an estimated channel has notsignificantly changed since CSI feedback was last transmitted.

Analysis

Expanding on the scenarios described in this disclosure document, a 40MHz IEEE 802.11 ac network is assumed with an 8-antenna AP and tendual-antenna stations experiencing a range of channel coherence times,such as 100 ms, 200 ms, 400 ms, 400 ms, 600 ms, 800 ms, 1000 ms and 1200ms. These channel coherence time values are consistent with recentmeasurement campaigns involving stationary stations in indoor conditionswith deliberate pedestrian activity in the channel (100 ms representsapproximately one percentile of the measurements). It is assumed that apreferred CSI feedback interval for a given station is ten percent ofits channel coherence time. In addition, a nominal uplink capacity of 54Mbps may be assumed for all the stations.

If the proposed protocol is not implemented, the system may be designedso that all stations transmit CSI feedback at a rate suitable for theexpected worst-case Doppler condition. Assuming 100 ms coherence time,all stations may therefore feedback CSI messages 100 times per second.Therefore, total capacity required for all CSI feedback messages may bewritten as follows:

$\begin{matrix}{{Capacity} = {N_{CSI} \times N_{b} \times N_{tx} \times N_{rx} \times N_{c} \times N_{sta} \times O_{MAC}}} \\{= {{100\mspace{14mu} C\; S\;{I/\sec} \times 16\mspace{14mu}{{bit}/C}\; S\; I \times 8 \times 2 \times 114 \times 10 \times 110\%} =}} \\{{30.6\mspace{14mu}{Mbps}},}\end{matrix}$where N_(CSI) may represent number of CSIs reported per second, N_(b)may represent number of bits used for reporting each CSI value, N_(tx)may represent number of transmit antennas, N_(rx) may represent numberof receive antennas, N_(c) may represent number of subcarriers, N_(sta)may represent number of stations and O_(MAC) may represent percentage ofMAC overhead. The required capacity (i.e., 30.6 Mbps) may beapproximately equal to 57 percent of the available 54 Mbps uplinkcapacity.

If the proposed protocol is implemented, CSI feedback may occur at arate appropriate for channel coherence time of each station. In theabove example, total throughput required for transmitting all the CSIfeedback messages may be equal to 8.3 Mbps, which representsapproximately 15 percent of the available 54 Mbps uplink capacity.Utilizing the proposed scheme may result in 73 percent reduction in thechannel overhead required for explicit CSI feedback compared to the casewhere the proposed techniques are not implemented.

In conditions where stations are subject to a range of SNRs or SINRs,further optimization may be possible by assigning lower feedback ratesto low MCS stations, resulting in additional overhead reduction.

The various operations of methods described above may be performed byany suitable means capable of performing the corresponding functions.The means may include various hardware and/or software component(s)and/or module(s), including, but not limited to a circuit, anapplication specific integrate circuit (ASIC), or processor. Generally,where there are operations illustrated in Figures, those operations mayhave corresponding counterpart means-plus-function components withsimilar numbering. For example, operations 500 and 600, 800, 900, 1100,1200, 1300 and 1400 illustrated in FIGS. 5, 6, 8, 9, 11, 12, 13, and 14respectively, correspond to means-plus-function blocks 500A, 600A, 800A,900A, 1100A, 1200A, 1300A, and 1400A illustrated in FIGS. 5A, 6A, 8A,9A, 11A, 12A, 13A, and 14A.

As used herein, the term “determining” encompasses a wide variety ofactions. For example, “determining” may include calculating, computing,processing, deriving, investigating, looking up (e.g., looking up in atable, a database or another data structure), ascertaining and the like.Also, “determining” may include receiving (e.g., receiving information),accessing (e.g., accessing data in a memory) and the like. Also,“determining” may include resolving, selecting, choosing, establishingand the like.

As used herein, the phrase “at least one of A or B” is meant to includeany combination of A and B. In other words, “at least one of A or B”comprises A or B or A and B.

The various operations of methods described above may be performed byany suitable means capable of performing the operations, such as varioushardware and/or software component(s), circuits, and/or module(s).Generally, any operations illustrated in the Figures may be performed bycorresponding functional means capable of performing the operations.

The various illustrative logical blocks, modules and circuits describedin connection with the present disclosure may be implemented orperformed with a general purpose processor, a digital signal processor(DSP), an application specific integrated circuit (ASIC), a fieldprogrammable gate array signal (FPGA) or other programmable logic device(PLD), discrete gate or transistor logic, discrete hardware componentsor any combination thereof designed to perform the functions describedherein. A general purpose processor may be a microprocessor, but in thealternative, the processor may be any commercially available processor,controller, microcontroller or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thepresent disclosure may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in any form of storage medium that is knownin the art. Some examples of storage media that may be used includerandom access memory (RAM), read only memory (ROM), flash memory, EPROMmemory, EEPROM memory, registers, a hard disk, a removable disk, aCD-ROM and so forth. A software module may comprise a singleinstruction, or many instructions, and may be distributed over severaldifferent code segments, among different programs, and across multiplestorage media. A storage medium may be coupled to a processor such thatthe processor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor.

The methods disclosed herein comprise one or more steps or actions forachieving the described method. The method steps and/or actions may beinterchanged with one another without departing from the scope of theclaims. In other words, unless a specific order of steps or actions isspecified, the order and/or use of specific steps and/or actions may bemodified without departing from the scope of the claims.

The functions described may be implemented in hardware, software,firmware or any combination thereof. If implemented in software, thefunctions may be stored as one or more instructions on acomputer-readable medium. A storage media may be any available mediathat can be accessed by a computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code in the form of instructions or datastructures and that can be accessed by a computer. Disk and disc, asused herein, include compact disc (CD), laser disc, optical disc,digital versatile disc (DVD), floppy disk, and Blu-ray® disc where disksusually reproduce data magnetically, while discs reproduce dataoptically with lasers.

Thus, certain aspects may comprise a computer program product forperforming the operations presented herein. For example, such a computerprogram product may comprise a computer readable medium havinginstructions stored (and/or encoded) thereon, the instructions beingexecutable by one or more processors to perform the operations describedherein. For certain aspects, the computer program product may includepackaging material.

Software or instructions may also be transmitted over a transmissionmedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition oftransmission medium.

Further, it should be appreciated that modules and/or other appropriatemeans for performing the methods and techniques described herein can bedownloaded and/or otherwise obtained by a user terminal and/or basestation as applicable. For example, such a device can be coupled to aserver to facilitate the transfer of means for performing the methodsdescribed herein. Alternatively, various methods described herein can beprovided via storage means (e.g., RAM, ROM, a physical storage mediumsuch as a compact disc (CD) or floppy disk, etc.), such that a userterminal and/or base station can obtain the various methods uponcoupling or providing the storage means to the device. Moreover, anyother suitable technique for providing the methods and techniquesdescribed herein to a device can be utilized.

It is to be understood that the claims are not limited to the preciseconfiguration and components illustrated above. Various modifications,changes and variations may be made in the arrangement, operation anddetails of the methods and apparatus described above without departingfrom the scope of the claims.

The techniques provided herein may be utilized in a variety ofapplications. For certain aspects, the techniques presented herein maybe incorporated in an access point station, an access terminal, a mobilehandset, or other type of wireless device with processing logic andelements to perform the techniques provided herein.

While the foregoing is directed to aspects of the present invention,other and further aspects of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A method for wireless communications, comprising:transmitting a first request message to request channel evolutionfeedback from a plurality of apparatuses; receiving channel evolutionfeedback from the apparatuses; determining a subset of the apparatusesthat should transmit new channel state information (CSI) feedback based,at least in part, on the channel evolution feedback; and transmitting asecond request message to the subset of apparatuses requesting new CSIfeedback.
 2. The method of claim 1, wherein the channel evolutionfeedback comprises at least one of a degree of channel aging since amost recent CSI update or a CSI feedback.
 3. The method of claim 1,further comprising: receiving one or more new CSI feedback messages fromthe subset of apparatuses; and updating precoding weights used fortransmissions to the subset of apparatuses, based on the new CSIfeedback messages.
 4. The method of claim 1, wherein the first and thesecond request messages are null data packet (NDP) announcement frames.5. The method of claim 1, wherein the determining the subset ofapparatuses comprises determining rate of channel evolution or Dopplerat the subset of apparatuses.
 6. The method of claim 1, wherein thedetermining the subset of apparatuses comprises comparing the channelevolution feedback with a previously obtained channel evolutionfeedback.
 7. The method of claim 1, further comprising: transmitting asounding message to the plurality of apparatuses after transmitting atleast one of the first or the second request messages, the soundingmessage comprising a preamble for channel sounding.
 8. The method ofclaim 7, wherein the sounding message is a null data packet (NDP) frame.9. The method of claim 1, wherein the receiving channel evolutionfeedback from the apparatuses comprises receiving channel evolutionfeedback transmitted from a at least two of the apparatusessimultaneously.
 10. The method of claim 1, wherein the second requestmessage comprises an indication of when each of the subset ofapparatuses should send CSI feedback.
 11. The method of claim 1, whereinthe CSI feedback comprises a representation of a current estimatedchannel.
 12. The method of claim 11, wherein the first request messagecomprises an indication of a type of CSI feedback being supported by arequesting device that transmitted the first request message.
 13. Themethod of claim 12, wherein the type of CSI feedback is multi-user (MU)or single user (SU).
 14. The method of claim 12, wherein the firstrequest message indicates whether differential updates to CSI feedbackare supported.
 15. The method of claim 1, wherein the CSI feedbackcomprises a representation of a relative change in an estimated channelsince previously received CSI feedback was transmitted.
 16. The methodof claim 15, wherein the CSI feedback comprises at least one of a Nulldata frame or an acknowledgement frame to indicate the estimated channelhas not significantly changed since CSI feedback was last received. 17.The method of claim 1, wherein the CSI feedback comprises an indicationof a type of CSI included in the CSI feedback.
 18. The method of claim17, wherein the type of CSI feedback is multi-user (MU) or single user(SU).
 19. An apparatus for wireless communications, comprising: atransmitter configured to transmit a first request message to requestchannel evolution feedback from a plurality of apparatuses; a receiverconfigured to receive channel evolution feedback from the apparatuses; acircuit configured to determine a subset of the apparatuses that shouldtransmit new channel state information (CSI) feedback based, at least inpart, on the channel evolution feedback; and wherein the transmitter isfurther configured to transmit a second request message to the subset ofapparatuses requesting new CSI feedback.
 20. The apparatus of claim 19,wherein the channel evolution feedback comprises at least one of adegree of channel aging since a most recent CSI update or a CSIfeedback.
 21. The apparatus of claim 19, wherein the receiver is furtherconfigured to receive one or more new CSI feedback messages from thesubset of apparatuses, and the apparatus further comprises circuitconfigured to update precoding weights used for transmissions to thesubset of apparatuses, based on the new CSI feedback messages.
 22. Theapparatus of claim 19, wherein the first and the second request messagesare null data packet (NDP) announcement frames.
 23. The apparatus ofclaim 19, wherein the circuit configured to determine the subset ofapparatuses is further configured to determine rate of channel evolutionor Doppler at the subset of apparatuses.
 24. The apparatus of claim 19,wherein the circuit configured to determine the subset of apparatuses isconfigured to compare the channel evolution feedback with a previouslyobtained channel evolution feedback.
 25. The apparatus of claim 19,wherein the transmitter is further configured to transmit a soundingmessage to the plurality of apparatuses after transmitting at least oneof the first or the second request messages, the sounding messagecomprising a preamble for channel sounding.
 26. The apparatus of claim25, wherein the sounding message is a null data packet (NDP) frame. 27.The apparatus of claim 19, wherein the receiver configured to receivechannel evolution feedback from the apparatuses is configured to receivechannel evolution feedback transmitted from at least two of theapparatuses simultaneously.
 28. The apparatus of claim 19, wherein thesecond request message comprises an indication of when each of thesubset of apparatuses should send CSI feedback.
 29. The apparatus ofclaim 19, wherein the CSI feedback comprises a representation of acurrent estimated channel.
 30. The apparatus of claim 29, wherein thefirst request message comprises an indication of a type of CSI feedbackbeing supported by the apparatus.
 31. The apparatus of claim 30, whereinthe type of CSI feedback is multi-user (MU) or single user (SU).
 32. Theapparatus of claim 30, wherein the first request message indicateswhether differential updates to CSI feedback are supported.
 33. Theapparatus of claim 19, wherein the CSI feedback comprises arepresentation of a relative change in an estimated channel sincepreviously received CSI feedback was transmitted.
 34. The apparatus ofclaim 33, wherein the CSI feedback comprises at least one of a Null dataframe or an acknowledgement frame to indicate the estimated channel hasnot significantly changed since CSI feedback was last received.
 35. Theapparatus of claim 19, wherein the CSI feedback comprises an indicationof a type of CSI included in the CSI feedback.
 36. The apparatus ofclaim 35, wherein the type of CSI feedback is multi-user (MU) or singleuser (SU).
 37. An apparatus for wireless communications, comprising:means for transmitting a first request message to request channelevolution feedback from a plurality of apparatuses; means for receivingchannel evolution feedback from the apparatuses; means for determining asubset of the apparatuses that should transmit new channel stateinformation (CSI) feedback based, at least in part, on the channelevolution feedback; and wherein the means for transmitting furthercomprises means for transmitting a second request message to the subsetof apparatuses requesting new CSI feedback.
 38. The apparatus of claim37, wherein the channel evolution feedback comprises at least one of adegree of channel aging since a most recent CSI update or a CSIfeedback.
 39. The apparatus of claim 37, wherein the means for receivingfurther comprises means for receiving one or more new CSI feedbackmessages from the subset of apparatuses, and wherein the apparatusfurther comprises means for updating precoding weights used fortransmissions to the subset of apparatuses, based on the new CSIfeedback messages.
 40. The apparatus of claim 37, wherein the first andthe second request messages are null data packet (NDP) announcementframes.
 41. The apparatus of claim 37, wherein the means for determiningthe subset of apparatuses comprises means for determining rate ofchannel evolution or Doppler at the subset of apparatuses.
 42. Theapparatus of claim 37, wherein the means for determining the subset ofapparatuses comprises means for comparing the channel evolution feedbackwith a previously obtained channel evolution feedback.
 43. The apparatusof claim 37, wherein the means for transmitting further comprises meansfor transmitting a sounding message to the plurality of apparatusesafter transmitting at least one of the first or the second requestmessages, the sounding message comprising a preamble for channelsounding.
 44. The apparatus of claim 43, wherein the sounding message isa null data packet (NDP) frame.
 45. The apparatus of claim 37, whereinthe means for receiving channel evolution feedback from the apparatusescomprises means for receiving channel evolution feedback transmittedfrom at least two of the apparatuses simultaneously.
 46. The apparatusof claim 37, wherein the second request message comprises an indicationof when each of the subset of apparatuses should send CSI feedback. 47.The apparatus of claim 37, wherein the CSI feedback comprises arepresentation of a current estimated channel.
 48. The apparatus ofclaim 47, wherein the first request message comprises an indication of atype of CSI feedback being supported by the apparatus.
 49. The apparatusof claim 48, wherein the type of CSI feedback is multi-user (MU) orsingle user (SU).
 50. The apparatus of claim 48, wherein the firstrequest message indicates whether differential updates to CSI feedbackare supported.
 51. The apparatus of claim 37, wherein the CSI feedbackcomprises a representation of a relative change in an estimated channelsince previously received CSI feedback was transmitted.
 52. Theapparatus of claim 51, wherein the CSI feedback comprises at least oneof a Null data frame or an acknowledgement frame to indicate theestimated channel has not significantly changed since CSI feedback waslast received.
 53. The apparatus of claim 37, wherein the CSI feedbackcomprises an indication of a type of CSI included in the CSI feedback.54. The apparatus of claim 53, wherein the type of CSI feedback ismulti-user (MU) or single user (SU).
 55. A computer-program product forwireless communications, comprising a computer-readable storage devicecomprising instructions executable for: transmitting a first requestmessage to request channel evolution feedback from a plurality ofapparatuses; receiving channel evolution feedback from the apparatuses;determining a subset of the apparatuses that should transmit new channelstate information (CSI) feedback based, at least in part, on the channelevolution feedback; and transmitting a second request message to thesubset of apparatuses requesting new CSI feedback.
 56. An access pointfor wireless communications, comprising: a plurality of antennas, atransmitter configured to transmit, via the plurality of antennas, afirst request message to request channel evolution feedback from aplurality of apparatuses; a receiver configured to receive channelevolution feedback from the apparatuses; circuit configured to determinea subset of the apparatuses that should transmit new channel stateinformation (CSI) feedback based, at least in part, on the channelevolution feedback; and wherein the transmitter is further configured totransmit a second request message to the subset of apparatusesrequesting new CSI feedback.